1. Field of the Invention
The present invention relates to a method and apparatus for determining the best legal flight altitudes, and the best points at which to change to a new legal flight altitude, such that the cost of the flight is minimized, but subject to filtering to prevent excessive changes in altitude. The method of computation is part of, and an improvement on, computations that are performed by existing Flight Management Systems for an aircraft.
2. Description of the Prior Art
Most commercial aircraft produced in recent years come equipped with a Flight Management System (FMS). An FMS accepts pilot-entered input data that represents a flight plan from the origin airport to the destination airport. A flight plan is composed mostly of a sequence of waypoints that define the horizontal flight path. Also specified is the initial cruise altitude, and for long flights, points at which to initiate climb to a new cruise altitude to take advantage of cost savings available when gross weight is reduced. In general, as fuel is burned off and the aircraft loses weight, the optimum cruise altitude (the altitude that minimizes cost of flight) increases.
A part of the computations performed by an FMS is therefore to compute the best altitude to fly so that the cost of the flight is minimized. The optimum altitude is defined as the altitude that minimizes cost (a combination of fuel cost and time cost, to be described below), and depends on aircraft gross weight, speed, wind and air temperature. However, the aircraft is usually constrained to fly at legal altitudes, established by the International Civil Aviation Organization (ICAO) in order to maintain traffic separation. The change from one legal altitude to another is referred to as a "step climb" or a "step descent". A step climb or descent may be taken only upon approval by Air Traffic Control. Step climbs are much more common than step descents since optimum altitude generally increases as fuel is burned off. Some systems prohibit computed step descents in the planned flight profile.
Altitude step points may be pre-planned and manually placed at any waypoint. However, the FMS also computes optimum step points, to inform the pilot of the best location to initiate a step. Optimum step points are generally not at the waypoints. As the aircraft approaches a pre-planned or an optimum step climb point, the pilot requests permission from Air Traffic Control to step at that point. Permission may be denied, or the step point may have to be moved, due to traffic interference.
In order to present to the pilot information about future points in the flight, such as arrival time and distance to go at future waypoints, and locations of future step climb points, a process called "prediction" is performed by the FMS. The prediction process simulates future flight by advancing a set of state variables, S, starting at the current aircraft state, along the flight plan, to the destination. A typical set of state variables are: time, distance to destination, altitude, gross weight, true air speed and flight path angle. Other sets can be used. The prediction is computed in fast time to provide downpath information to the pilot as quickly as possible. It is performed by advancing S in "Prediction Intervals" that vary in size depending on the flight conditions (e.g. climbing at speed, level flight at speed, accelerating in climb). Speed at future points is obtained from a "speed generator" that gives the target speed for the selected speed mode. The "Econ" speed is designed to minimize cost, and uses gross weight, altitude, tail wind and a cost index (to be described below) as inputs. Tail wind is obtained from forecast wind data entered at waypoints in the flight plan. The prediction assumes that the steps will be taken as planned or computed.
In current systems, optimum altitude is typically computed using a lookup table that has been derived off-line by aerodynamic analysis for the specific aircraft, and is supplied as fixed data in the FMS software. This data table is based on the International Standard Atmosphere (ISA), with no winds, and with no deviations from the ISA temperature profile. With this table and with gross weight and speed (or equivalent variables) as input, the optimum altitude is computed as output. The method of computation is quick. Gross weight at future points is obtained from the prediction process. The speed generator is used to obtain aircraft speed for future points in the prediction process as well as for the current aircraft guidance function.
While forecast winds and temperature deviations are used in doing the prediction, they are not used in computing optimum altitude. Accordingly, the table lookup method for finding the optimum altitude is feasible (ignoring wind) because the function is then continuous and well behaved. The optimum altitude without wind increases nearly linearly with distance as fuel is burned off. The optimum step climb point is then determined by the distance point where the optimum altitude lies half way between the current cruise altitude and the next legal altitude. A typical Flight Profile using this method is shown in FIG. 1, to be described below. FIG. 1 also shows maximum and optimum altitude profiles verses distance.
A problem is encountered in present systems because wind is not taken into account. Thus, the FMS will call for the predicted altitude change at the step climb or descent point even if the new altitude would result in less economical flight. For example, if the wind at the called-for altitude is an extream head wind, the system will not compute an alternate altitude where more favorable wind conditions exist.